Various types of hydrostatic continuously variable transmissions have been known and commercialized. Some examples are hydrostatic continuously variable transmissions disclosed in Patent Documents 1, 2, and 3 which the present applicant proposed. The hydrostatic continuously variable transmissions disclosed in these patent documents include a swash plate plunger pump, a swash plate plunger motor, and a hydraulic closed circuit connecting the discharge port and intake port of the swash plate plunger pump to the discharge port and intake port of the swash plate plunger motor, where a pump swash plate member is driven by an engine and a pump cylinder and a motor cylinder are joined and disposed together on an output shaft, rotation of a motor swash plate member is controlled and the motor swash plate angle is adjustable.
It has been known that in this type of hydrostatic continuously variable transmission, a clutch valve which connects or disconnects the high pressure oil path and low pressure oil path which constitute the hydraulic closed circuit is provided and clutching operation is done to control the magnitude of rotational driving force from the hydraulic pump to be transmitted to the hydraulic motor or stop this rotational force transmission. For example, Patent Document 3 discloses an automatic clutch device which uses a clutch valve like this. This clutch valve includes a spring (biasing means) for biasing it toward the opening direction and a hydraulic governor for generating a governor oil pressure corresponding to input rotating speed, and opens or closes according to the spring biasing force and the governor force (governor oil pressure) so as to connect or disconnect the high pressure oil path and the low pressure oil path.
[Patent Document 1] JP-A No. 42446/1994
[Patent Document 2] JP-B No. 2920772
[Patent Document 3] JP-A No. 100909/1997
In the above clutch valve, when the engine rotating speed (input rotating speed) is low (for example, during engine idling), the spring biasing force prevails and opens the clutch valve; and when the engine rotating speed is high, the governor force prevails and closes the clutch valve. However, the problem is that at the time of start of a vehicle when the clutch is partially engaged, namely the clutch valve is partially open, it is necessary to add an oil cooling device or improve the performance of an oil cooling device because only some of the engine power is used as a driving force and the rest of the engine power might cause the working oil temperature in the hydraulic closed circuit to rise.